1. Field of the Invention
The present invention is related generally to the field of light emitting diode devices, and more specifically to an architecture for an improved high-Al content, low defect heterostructure quantum well surface emitting light emitting diode device.
2. Description of the Prior Art
In the III-V compound semiconductor family, the nitrides have been used to fabricate visible wavelength light emitting diode active regions. They also exhibit a sufficiently high bandgap to produce devices capable of emitting light in the ultraviolet, for example wavelengths between 300 and 400 nanometers. In particular, InAlGaN systems have been developed and implemented in visible and UV spectrum light emitting diodes (LEDs), such as disclosed in. U.S. Pat. No. 6,875,627 to Bour et al., which is incorporated herein by reference. These devices are typically formed on an Al2O3 (sapphire) substrate, and comprise thereover a GaN:Si or AlGaN template layer, an AlGaN:Si/GaN superlattice structure for reducing optical leakage, an n-type electrode contact layer, a GaN n-type waveguide, an InGaN quantum well heterostructure active region, and a GaN p-type waveguide region. In addition, the complete device may also have deposited thereover a p-type AlGaN:Mg cladding layer and a capping layer below a p-type electrode.
While significant improvements have been made in device reliability, optical power output, and mode stability, the performance of the nitride-based light emitting diode emitting in the ultraviolet (UV) is still far inferior to that of blue or green light emitting diode. It is particularly true that for the UV light emitting diodes, the nature of the substrate and template layer have a critical impact on the overall device performance. For example, electrical resistance between the structural layers of the device significantly effects optical output. While Al2O3 (sapphire) as a substrate has numerous advantages, the AlGaN template layer formed over the typical Al2O3 substrate posses high series resistance due to limited doping capabilities. Furthermore, the crystallographic structure of the device layers plays a key role in the device's operational characteristics, and the AlGaN template layer provides a relatively poor crystalline template.
The dislocation densities in AlGaN or AlN template layers on sapphire are typically in the mid 109 to high 1010 cm−2 range. As a consequence, the external quantum efficiencies of deep UV light emitting diodes in the 250 nm to 350 nm range are still below 2% even for the very best devices (external quantum efficiencies near 50% have been demonstrated for blue GaN-based LED structures). The high dislocation densities in AlGaN or AlN template layers on sapphire also pose significant problems for the light emitting diode device lifetimes.
GaN epitaxial layers on sapphire substrates have proven to be a better template for InGaAlN film growth, providing excellent optoelectronic quality for visible light emitting diode devices and reasonable dislocation densities. The dislocation densities in GaN template layers on sapphire are typically in the low 109 to mid 107 cm−2 ranges. Accordingly, sapphire with a GaN template layer is the preferred foundation for visible GaN-based light emitting diodes.
The output wavelength of the light emitting diode is inversely related to the Al content in the multiple quantum well heterostructure (MQWH) active region of the device. Thus, in order to obtain shorter wavelength devices, such as those emitting in the UV, the Al content of the HQWH region must be increased over that found in devices emitting in the visible spectrum. However, increasing the Al content presents a number of structural and device performance problems.
Furthermore, efforts to improve the quality of the LED structure in the ultraviolet range on GaN/sapphire template have presented significant challenges due to the large lattice mismatch between the epitaxial layers formed over the GaN crystallographic template which is known to lead to strain-induced cracking. This lattice mismatch is exacerbated when the Al content of layers formed above the GaN/sapphire system increases. Yet, as previously mentioned, an increased Al content (e.g., up to ˜50% in the MQWH active region of a 280 nm light emitting diode, and 60% to 70% in the surrounding AlGaN current and optical confinement layers) is required to obtain devices which emit in the UV. A UV InAlGaN heterostructure grown on GaN/sapphire is under tensile stress, which causes cracking of the AlGaN epitaxial layers when the critical layer thickness is exceeded. The critical thickness for an AlGaN film with a 50% aluminum mole fraction is about 20-50 nm, which is much too thin for realizing a usable device structure in the deep UV. Efforts to provide strain relief to accommodate the lattice mismatch have heretofore proven unsuccessful or impractical.
Various groups have published approaches to dealing with these shortcomings. For example, Han et al., Appl. Phys. Lett, Vol 78, 67 (2001), discuss the use of a single AlN interlayer formed at low temperatures to avoid strain development. This low-temperature AlN interlayer approach has proven unsuccessful in the case of heterostructure growth with high Al mole fractions. Nakamura et al., J. J. Appl. Phys., vol. 36, 1568 (1997) has suggested short period GaN/AlGaN superlattice layers as a way of extending the critical layer thickness of AlGaN films grown pseudomorphically on GaN/sapphire. But the average Al mole fraction in these AlGaN/GaN systems is at such a low level (˜10% or less) that it is not compatible with deep UV light emitting diodes. Finally, Chen et al., Appl. Phys. Lett., vol. 81, 4961 (2002) suggests an AlGaN/AlN layer as a dislocation filter for an AlGaN film on a AlGaN/sapphire template. But again, the AlGaN/sapphire template presents the aforementioned series resistance problem. There is a need for a deep UV light emitting diode apparatus with improved operation characteristics, and therefore, there is a need for a method and structure facilitating a high Al content MQWH active region which is free from cracking and related damage.